This invention relates to a method and apparatus for controlling the magnitude of the electric charge downstream of the last stage of a steam turbine and thereby decreasing the harmful effect of the electric charge on steam turbine operation, resulting in an increase in the power generated by the steam turbine.
The appearance of moisture in the steam flow of a steam turbine is accompanied by the creation of electric charges on water droplets. Depending on the feed water chemical composition and the material of the blades and vanes, the sign and magnitude of charges on the droplets may be subject to change. As a rule, the water droplets in the turbine flow paths are charged positively. Charges on the droplets are formed mainly due to separation of the double electric layer when droplets break off from the turbine blades and other metal surfaces. The charged droplets, in being driven under the effect of an inhomogeneous electric field toward the hood""s grounded surfaces, create an additional turbulence and resistance to the steam flow, thereby increasing the power losses to result in a decrease of the turbine unit efficiency. If the droplets carry away a positive charge, the turbine blades and hence the rotor acquire a negative charge. The presence of a negative potential on the turbine rotor may cause erosion of bearings, unless special grounding is provided. Separation of charges when the droplets break off from the blades may be accompanied by the dissociation of water molecules due to the increasing electric field and by the appearance of hydrogen. If the liberated hydrogen reacts with the metal, hydrides may appear, which make the metal brittle. Besides, the electric discharges appearing between the charged droplets and the grounded metal surfaces lead to metal destruction. All this in turn causes increased erosion of the internal elements and the hood surfaces.
The efficient operation of a turbine-generator set depends essentially on the steam moisture content in the last stages of a turbine. Typically, an increase of moisture content by 1% causes a drop in the turbine stage efficiency by approximately 1%. At the same time, an increase of the moisture content causes increased erosion of the turbine blades and vanes as well as of the surfaces of the turbine exhaust hood elements. During operation of a turbine, electric charges are formed in the moving steam flow. These charges appear on liquid water droplets, whereas dry steam is substantially without electrification. These processes are described in U.S. Pat. No. 5,735,125, which is incorporated by reference and in U.S. patent application Ser. No. 09/037,902 which is also incorporated by reference.
The presence of charged droplets in the steam flow causes volume condensation of saturated vapor as in the well-known Wilson Chamber used to visualize the tracks of subatomic particles. The process has an avalanche character since volume condensation contributes to an increase of the steam moisture content, which in turn is accompanied by an increase of the volume charges.
The process of formation of electric charges depends on a number of physical factors, including, flow velocity, moisture content, the surface condition of the turbine blades and the turbine flow guides, as well as on the intensity and the polarity of the electric field. Downstream of the last turbine stage, the steam flow possesses significant kinetic energy. In the presence of water droplets in the steam flow, the, turbine acts as an electrostatic aerosol generator. Electrostatic aerosol generation may reach 4% and more of the mechanical power. Part of the steam flow kinetic energy is spent on creating an electric field which is especially powerful near the last stage. This field is directed in opposition to the steam flow, resulting in braking of the charged droplets. In addition, the charge density and the electric field intensity both in the axial and in the radial directions are highly irregular. Such an electric field may create electric charges adjacent to the grounded parts of the steam flow guides. The electric field drives the charged droplets crosswise to the flow towards the grounded surfaces of the exhaust hood, thereby creating additional resistance to the flow. All this leads to intensification of the turbulent flow and an increase of losses in the hood, resulting in a reduction of the generating unit""s energy conversion efficiency.
In U.S. patent application Ser. No. 09/037,902 it has been suggested to effect the wet steam flow charge and thereby increase power output by changing the feed water chemical composition as well as by selecting the material of the blades and flow guides working surfaces. In a number of cases this method may prove to be fairly convenient since it allows the steam flow charge over the turbine""s entire steam flow path without using additional devices. However, due to different quality of water used at power plants, this method may not be used at all times.
The invention disclosed herein is distinct from that disclosed in U.S. Pat. No. 5,735,125 in structure, operation and mechanism. In U.S. Pat. No. 5,735,125 (1) the active electrodes are installed above the condenser tube bundles, (2) the active electrodes alternate with grounded counterelectrodes, (3) a voltage of the same polarity as the sign of the electric charge present in the steam is applied to the active electrodes, (4) the voltage is adjusted as high as can be maintained without causing arcing or sparking between electrodes, and (5) the process is believed to increase power output by decreasing turbulence in the steam flow at the entrance to the condenser and increasing the density of condensation nuclei. In the invention disclosed herein (1) the active electrodes are located close to the last stage turbine wheel, (2) no counterelectrodes are provided, (3) the voltage applied to the counterelectrodes has polarity opposite to the sign of the charge present in the steam flowing out of the turbine, (4) the voltage is adjusted to give approximately zero net charge density in the flow downstream of the electrodes, and (5) the beneficial effects flow from eliminating electric charge from the flow whereby turbulence in the exhaust hood is decreased, increasing power output.
The method of the invention includes the step of receiving, at a steam turbine final stage, a steam byproduct with an electrically charged component. An electric field is created immediately proximate to the steam turbine final stage to substantially eliminate the electrically charged component from the steam byproduct and thereby reduce turbulence associated with the steam byproduct.
The apparatus of the invention is a steam turbine with a final stage turbine wheel to eject a steam byproduct with an electrically charged component A set of active electrodes create an electric field immediately proximate to the final stage turbine wheel. The electric field operates to substantially eliminate the electrically charged component from the steam byproduct and thereby reduce turbulence associated with the steam byproduct.
The invention reduces the turbulence and the backpressure downstream the last turbine stage, which reduces losses due to moisture content, thereby increasing power output, and minimizing erosion of the structural elements and surface of the exhaust hood in steam turbines.